A. Field of the Invention
The present invention relates to apparatus for use in the drilling and operation of sub-surface wells, particularly oil wells and geothermal wells. More particularly, the invention relates to an improved annular sealing element for use in blowout preventers of the type used to prevent pressurized subterranean liquids or gases from blowing out and upwards through a well hole.
B. Discussion of Background Art
In drilling for natural gas or liquid petroleum, a drill string consisting of many lengths of threaded pipes screwed together and terminated by a drill bit head is used to bore through rock and soil. The drill bit head has a larger diameter than the pipes forming the drill string above it. The upper end of the drill string is rotated to transmit a rotary boring action to the drill bit head.
During the drilling operation, a specially formulated mud is introduced into an opening in an upper drill pipe. This mud, which typically is selected to have a high specific gravity, flows downwards through the hollow interior of the pipes in the drill string and out through small holes or jets in the drill bit head. Since the drill bit head has a larger diameter than the drill string above it, an elongated annular space is created during the drilling process which permits the mud to flow upwards to the surface. Mud flowing upwards carries drill cuttings, primarily rock chips, to the surface. The mud also lubricates the rotating drill string, and provides a downward hydrostatic pressure which counteracts pressure which might be encountered in subsurface gas pockets.
In normal oil well drilling operations, it is not uncommon to encounter subsurface gase pockets whose pressure is much greater than could be resisted by the hydrostatic pressure of the elongated annular column of drilling mud. To prevent the explosive and potentially dangerous and expensive release of gas and/or liquid under pressure upwards out through the drilling hole, blowout preventers are used. Blowout preventers are mounted in a pipe casing surrounding a drill hole, near the upper end of the hole.
Typical blowout preventers have a resilient sealing means which can be caused to tightly grip the outer circumferential surfaces of various diameter drill string components, preventing pressure from subterranean gas pockets from blowing out material along the drill string. Usually, the resilient sealing means of a blowout preventer is so designed as to permit abutting contact of a plurality of sealing elements, when all elements of a drill string are removed from the casing. This permits complete shutoff of the well, even with all drill string elements removed. Most oil well blowout preventers are remotely operable, as, for example, by a hydraulic pressure source near the drill hole opening having pressure lines running down to a hydraulic actuator cylinder in the blowout preventer.
Blowout preventers having resilient sealing means are disclosed in U.S Pat. No. 3,323,773, R. W. Walker, Jun. 6, 1967, and U.S. Pat. No. 3,667,721, issued Jun. 6, 1972 to A. N. Vujasinovic.
Prior blowout preventers, including those disclosed in the above-identified U.S. patents, typically use a circularly spaced array of curved metal segments which are contained slidably in a hemispherical cavity and pushed upwards by a hydraulic piston to effect a reduction in diameter of an upward entrance bore to the spherical cavity, through which drill string components are inserted. The curved metal segments are held in a circumferentially spaced relationship by being molded integrally into a resilient rubber matrix having a generally cylindrical interior shape. When the sealing element comprising the curved metal segments and resilient matrix are moved upwards, the inner cylindrical rubber surface is forced to cold flow inwards towards the outer circumferential surface of the drill stem components within the blowout preventer, thereby effecting a seal and preventing pressurized fluids below the blowout preventer from escaping upwards. In some prior art blowout preventers, sufficient movement of rubber inwards is afforded to completely seal the bore through the blowout preventer, even with all drill string components withdrawn.
Existing blowout preventers can damage drill string components under certain conditions. Since the metal segments used in the sealing element of some blowout preventers is non-resiliently translated longitudinally upward and radially inward by the actuator piston, the upper inner edges of the segments can contact the circumferential surface of drill string components with radial compressive forces sufficient to damage the component. With this and other limitations of prior blowout preventers in mind, the present inventors developed an improved blowout preventer which incorporates force-limiting means for preventing rigid metal segments in the annular sealing element of the blowout preventer from being forced against drill string components sufficiently hard to damage those components. The improved blowout preventer was disclosed in U.S. patent application Ser. No. 054,932 filed 05/27/87 and titled Blowout Preventer With Radial Force Limiter. That application resulted in the issuance on Aug. 22, 1989 of U.S. Pat. No. 4,858,882 with the same title.
One embodiment of applicants' above-referenced Blowout Preventer With Radial Force Limiter uses an annular sealing element in which metal sealing elements are moved in front of resilient elastomeric material interposed in the path extending from an actuating piston to the metal sealing elements and thence to the outer cylindrical surface of a drill string component within the bore of the blowout preventer. The compressibility of the resilient material limits the impact force exertable by the rigid sealing elements against the drill string component.
A novel sealing element used in another embodiment of the improved blowout preventer referenced above uses metal sealing segments made of two parts which are moveable with respect to one another. The freedom of one part of a metal sealing segment to move with respect to the other part of the segment limits the force which the metal segments may exert on drill string components as the actuating piston of the blowout preventer moves the sealing element into a closing position.
The present invention was conceived of to provide an improved sealing element for blowout preventers. The improved sealing element includes means for limiting the radial force which the sealing element may exert on a drill string component, and also includes means for compensating for wear of resilient portions of the sealing element, thus maintaining sealing effectiveness for a greater number of operational cycles.